1. Field of the Invention
This invention relates to cordless telephones. More particularly, the invention concerns a method and apparatus for detecting errors in the transmission of digital command words used to control communication protocol between the base and handset of a cordless telephone set.
2. Background Discussion
Cordless telephones normally consist of two separate stations, a base station and a handset station. The base station is plugged into a telephone outlet and comprises all the necessary equipment and electronics for interfacing with the local telephone trunk. The handset station comprises all the electronics and equipment necessary for interfacing with the user of the telephone. Such equipment includes a touch-tone key pad for dialing telephone calls and providing other features, as well as a speaker and microphone. Both stations also include radio transceivers and antennae for radio communication between the handset and the base station.
Since the base station is essentially stationary and usually must be hardwired to a telephone outlet, it will typically receive its electrical power supply from an AC wall outlet. The telephone handset, however, is intended to be portable and therefore cannot be designed for connection to an AC power outlet. Therefore, the handset receives its power from an on-board battery pack. Typically, the batteries are of the rechargeable type, and are recharged when the handset is placed in an accepting cradle on the base station and electrically coupled to the base station power supply via a recharge circuit.
In addition to the actual conversation that takes place on the telephone, digital signals for governing protocol between the two separate stations are also transmitted. Typically, protocol control is carried out by transmitting digital command words and digital acknowledgement words (digital data words, collectively) between the two stations. This may be accomplished on a separate frequency or channel than the voice communications or may be accomplished directly over the voice frequency channels. When transmitting digital words through the air via radio signals, there is a possibility that the transmitted data may not be received correctly at the receiving station. Unfortunately, there is a substantial amount of radio frequency interference and noise which could effect the integrity of the transmitted data, particularly as the distance between the base station and handset increases.
In order to transmit the protocol data words from one station to another, the words are multiplexed onto a single channel and the bits are sent sequentially through the air to the receiving station s antenna. Most noise and interference which effects the integrity of data transmissions in cordless telephone systems consists of bursts of interference of varying duration. An interference burst may effect the validity of only one or two received bits, if the interference is of particularly short duration, or of the entire word if it is of longer duration.
Prior art cordless telephone sets sometimes appended a security code to the protocol data word in order to provide an error detection method and a source identification method. In these cordless telephones, the security code word is stored in the memory of both the base station and the handset station. When a protocol transmission is necessary between stations, the security code is appended to the protocol word to form a validation code which is then transmitted. For instance, if protocol signalling is accomplished by using an eight bit protocol word, instead of simply sending the eight bit protocol command word, the transmitting station sends a sixteen bit word consisting of the protocol command as the high byte and the security code as the low byte. The receiving station reads the transmitted word and compares the security byte portion of it with the security code that it has stored in its memory. The security byte also serves to provide source identification to the receiving station. Each telephone set, comprising a base station and a handset station, is programmed with a different security code, thereby preventing two telephone systems located in the same transmitting vicinity from causing crosstalk between the two separate systems. A transmission from the handset or base of one system will not be accepted by the other system because it will not recognize the security code. If there is a match, then it is likely that the entire transmitted word has been received correctly and the receiving station accepts the protocol word as being correct. This type of error detection, however, is far from perfect. Short duration interference bursts causing errors can go entirely undetected. For instance, in the above example of an eight bit security code and an eight bit protocol word, errors up to eight bits long can go entirely undetected. If an eight bit burst error occurs during the protocol command portion of the transmitted word rather than the security code portion, the security code would be received correctly and the receiving station would accept the protocol word as correct.
In addition to noise and random interference, cordless telephones are particularly susceptible to intentional "false" transmissions. Particularly, unauthorized use of the base station of a cordless telephone set can be made from other transceivers. Another person knowing the transmitting frequency and protocol system of the cordless telephone set, can use a transceiver to simulate the handset of the cordless telephone and make unauthorized calls using the base station of the cordless telephone or tap into private telephone conversations.
Therefore, it is an object of the present invention to provide an improved cordless telephone set.
It is another object of the present invention to provide a cordless telephone in which it is extremely difficult to make unauthorized telephone calls using a remote transceiver.
It is a further object of the present invention to provide an improved means for detecting transmission errors in a cordless telephone.
It is still another object of the present invention provide a cordless telephone set that can correct for certain transmission errors.